The cause of persistent and disabling vertigo and disequilibrium following vestibular injury is not known. The objective of the proposed research is to improve the treatment of patients with balance disorders by providing new information about the response of the vestibular system to neurotological surgery. The specific aims of this project are first, to test the hypothesis that the site of acute vestibular injury, whether limited to the vestibular neuroepithelium and pre-ganglionic vestibular nerve or involving post- ganglionic vestibular nerve injury, influences the recovery of vestibular reflexes. The second specific aim is to test whether the time course of vestibular injury, acute versus gradual, influences the recovery of vestibular reflexes. The third specific aim is to begin to investigate the mechanisms of compensation by studying acute and long-term cellular and transneural changes within the central vestibular nuclei. These specific aims will be tested in a cat animal model of vestibular neurotological surgery. The first two specific aims will examine recovery of vestibular reflexes following surgery. To accomplish these aims, we will examine the horizontal vestibulo-ocular reflex (HVOR), the otolith- ocular reflex (OOR), the vestibulo-collic reflex (VCR), and gaze stabilization during combined eye-head movements. The final specific aim involves a quantitative histopathologic analysis of the central vestibular nuclei in these same animals utilizing silver degeneration stains, cytomorphometrics, and immunocytochemistry for the astroglial specific protein glial fibrillary acidic protein (GFAP). This research will increase our understanding of the response of the vestibular system to ablative vestibular lesions. The significance of this research is that the data from the proposed studies will provide basic scientific information to clinicians which may enable them to select ablative procedures which yield the greatest functional recovery.